Coriolis type gyroscopes, fabricated using Micro-Electro-Mechanical Systems (MEMS) technology, are now widely used for a range of high volume applications. For example, in the automotive industry such gyroscopes can be utilised in advanced braking systems, active suspension, rollover detection and prevention applications and navigation applications. The performance requirements of gyroscopes for such applications are relatively undemanding, particularly when compared to typical aerospace and military applications, where gyroscopes are used for guidance and control.
The suitability of MEMS gyroscopes for such applications is largely based on their low cost per unit, which is achieved primarily by the wafer level fabrication techniques of MEMS processing that enables MEMS gyroscopes to be economically produced in large quantities. The performance level of MEMS gyroscopes is relatively modest in comparison with other classes of gyroscope, such as spinning wheel or fibre optic gyroscopes. However, the use of higher specification gyroscopes is typically restricted to aerospace and military applications as the unit cost of such gyroscopes is too high to be considered for higher volume automotive and commercial applications. Conversely, the lower performance capability of MEMS gyroscopes prohibits their use in the majority of aerospace and military applications.
UK patent GB2322196 describes a MEMS gyroscope that is used for automotive and commercial applications. The low cost and rugged nature of this MEMS gyroscope makes it attractive for use in guidance and control, navigation and platform stabilisation applications. However, the performance capability, particularly in terms of the rate bias stability and signal to noise ratio, is typically insufficient to meet these requirements. It would therefore be advantageous to enhance the performance of such prior art apparatus.
Referring to FIG. 1, a known MEMS gyroscope 10, similar to that described in UK patent GB 2322196, includes a sensing element comprising a planar silicon ring structure 12 that is externally supported by eight compliant legs 14a to 14h. These legs 14a to 14h are in turn attached to a rigid external frame 16. The gyroscope 10 is arranged to operate using a cos 2θ vibration mode pair, as illustrated with referenced to FIGS. 2a and 2b. One of the vibration mode pair is excited as a primary carrier mode P, FIG. 2a wherein the dotted line indicates the extremes of motion of the ring structure 12 in the primary carrier mode P. When the ring structure 12 is rotated around an axis 18 normal to the plane of the ring structure 12 Coriolis forces are generated, which couple energy into a secondary response mode S, FIG. 2b wherein the dotted line indicates the extremes of motion of the ring structure 12 in the secondary response mode S. The amplitude of the induced motion is directly proportional to the applied rotation rate about the axis normal to the plane of the ring structure 12.
The primary carrier mode P is generated using at least one drive transducer and the secondary response mode S is detected using at least one pick-off transducer. The drive and pick-off transducers are arranged around the ring structure 12.
Referring to FIG. 3, in order to implement both drive and pick-off transducers, a metal track is provided on an upper surface of a ring structure 20 and an external frame 22, over an insulating surface oxide layer. Referring to FIG. 4, wherein in like references have been used to indicate similar integers to those described with reference to FIG. 3, metal track 24 is repeated for eight circuits, one circuit for each drive and pick-off transducer. Each circuit of metal track 24 begins at a first bond-pad 26, runs along a first compliant leg 28, across an eighth segment 30 of the ring structure 20 and back along an adjacent compliant leg 32 to a second bond-pad 34. This circuit arrangement is repeated for each eighth segment 30 of the ring structure 20, such that each compliant leg 28 carries a metal track 24 for two adjacent transducers.
A third metal track 36 is located along the centre of each compliant leg 28, 32, between the metal track 24 associated with adjacent transducers to reduce the cross-coupling effect between circuits associated with adjacent transducers.
A magnetic field B is applied around the periphery of the ring structure 20 and arranged perpendicular to the plane of the ring structure 20. The magnetic field is applied by a permanent magnet 38, located inside the circumference of the ring structure 20, and an upper pole piece 40 and lower pole piece 42 which are arranged to concentrate the magnetic field in a gap between the pole pieces 40, 42, in the vicinity of the periphery of the ring structure 20.
The ring structure 20, compliant legs 28, 32 and external frame 22 are bonded onto a supporting glass substrate 44. This assembly is in turn bonded onto a glass support structure 46 together with the permanent magnet 38 and pole pieces 40, 42, which are assembled with the ring structure 20 located in the gap between the upper and lower pole piece 40, 42.
In operation, passing an alternating current through a metal track 24 will generate a Lorentz force where the metal track 24 passes through the magnetic field. The magnitude of the force, FI, will be given by:FI=BIL  Equation (1)
where B is the magnetic field, I is the current and L is the length of metal track 24 in the magnetic field. If the frequency of the alternating current is at the resonance frequency of the primary carrier mode, the ring structure 20 will be excited into resonant vibratory motion. A metal track 24 arranged in this manner will define a drive transducer.
Where a metal track 24 is in motion within the magnetic field, a voltage, V, will be generated across the metal track 24, which is given by:V=vBL  Equation (2)
where v is the peak velocity of the vibratory motion of the metal track 24 in the magnetic field, B is the magnetic field and L is the length of metal track 24 in the magnetic field. A metal track 24 arranged in this manner will define a pick-off transducer.
Such a MEMS gyroscope 10 will typically be operated in a closed loop mode of operation. In this mode, the primary carrier mode P is driven at the resonance maximum using a primary drive transducer controlled by a Phase Locked Loop. The amplitude of motion of the ring structure 12 is maintained at a constant value by an Automatic Gain Control loop, which is arranged to compare the amplitude of motion of the ring structure 12, as measured at a primary pick-off transducer, to a fixed reference level and to dynamically adjust a drive signal to the primary drive transducer to maintain a constant signal level and hence a constant amplitude of motion of the ring structure 12. The magnitude of the Coriolis force induced when the ring structure 12 is rotated about the axis 18 normal to the plane of the ring structure 12, and hence the scale factor of the MEMS gyroscope 10 is directly proportional to the amplitude of the primary carrier mode P motion. The Coriolis force will induce motion in the secondary response mode S, which is detected by a secondary pick-off transducer. In the closed loop mode of operation, the secondary response mode S motion is nulled by an appropriately controlled secondary drive transducer. The drive force required to maintain the null condition of the secondary response mode S provides a direct representation of the rotation rate applied about the axis 18.
Generally, there are a number of possible sources of rate bias error for a MEMS gyroscope 10. The two most significant errors for a MEMS gyroscope 10 are known to be the quadrature bias error, which arises due to imperfections in the geometry of the ring structure 12, and the cross-coupling error, which arises due to direct coupling of the primary drive signal into the secondary pick-off signal detected in a rate channel.
In an ideal case, wherein the electronic circuits are suitably phased and there are identical carrier and response mode frequencies for the ring structure 12, there will be no motion detected at a secondary pick-off transducer when the MEMS gyroscope 12 is not subject to rotation about axis 18. However, in reality, small geometric imperfections of the ring structure 12, arising during the fabrication process, will give rise to a small splitting of the primary carrier P and secondary response S mode frequencies. This frequency split also tends to fix the angular position of the primary carrier P and the secondary response S modes at an arbitrary angle, , with respect to a primary drive transducer arranged at =0°. Accordingly, if the arbitrary angle is not zero,  0°, a primary drive force applied by the primary drive transducer will excite both the primary carrier P and secondary response S modes to some extent. A phase locked loop circuit associated with the primary drive transducer will adjust the drive frequency to achieve a 90° phase shift between the primary carrier P and the secondary response S modes. However, motion will exist along the axis of the secondary response mode S that will be predominantly in quadrature phase with respect to the primary carrier mode P. In a closed loop system this motion will be nulled by a quadrature force component applied by the secondary drive transducer.
The quadrature drive level required to null the motion along the axis of the secondary response mode S is referred to as the quadrature bias, QUAD, and is defined as:ΩQUAD=K×ΔF×sin 4α  Equation (3)
where F is the mode frequency split,  is the mode angle with respect to the primary drive axis and K is a constant including terms for the modal coupling coefficient and the secondary drive and primary pick-off gains.
Quadrature bias, QUAD, can be large in comparison to the rate signals that the MEMS gyroscope is required to measure. A typical rate measurement range for a MEMS gyroscope 10 used in an automotive application is ±100°/sec. The quadrature bias, QUAD, if scaled in degrees per second, can be large, for example greater than ±100°/sec, in comparison to the rate signal and can vary significantly over the operating temperature range of the MEMS gyroscope 10. Where a phase error, E, exists, a small proportion of this error signal will appear on the rate channel. This will give rise to a rate bias error, Err, which is given by:ΩErr=K×ΔF×sin 4α×sin φE  Equation (4)
Even a relatively small phase error, E, can give rise to a significant rate bias error, Err. Any variation in the quadrature signal or the phase error, E, with temperature will cause the quadrature bias, QUAD, to vary which will severely limit the accuracy and stability of the MEMS gyroscope 10. The phase error, E, will typically be relatively stable over the operating temperature range of the MEMS gyroscope 10. However, the quadrature bias, QUAD, will typically vary by a significant amount. This large variation results in a correspondingly large variation in the quadrature bias error and hence an instability in the rate bias performance.
The dominant contribution to the cross-coupling of the primary drive signal into the secondary pick-off, and hence the rate channel, arises due to the close proximity of the metal tracks 24 associated with adjacent transducers, which run parallel to one another along a single compliant leg 28, 32. A third metal track 36 connected to ground is arranged between the two parallel metal tracks 24 to minimise the direct capacitive coupling between the metal tracks 24. However, a significant electrical coupling still exists between the two metal tracks 24. The magnitude of the cross-coupling bias error, C, arising from this coupling is defined as:
                              Ω          C                =                              K            ×            F            ×                          ϕ              C                                            Q            2                                              Equation        ⁢                                  ⁢                  (          5          )                    
where C is the coupling coefficient, F is the resonance frequency, and Q is the Quality Factor of the resonance mode. This mechanism is known to contribute to rate bias, for example between 1 and 2 degrees per second of rate bias at ambient temperature. This value will scale as 1/Q2 and the Quality Factor, Q, will vary by a significant amount, i.e. ±50%, over the operating temperature range of the MEMS gyroscope 10. Therefore, this mechanism makes a significant contribution to the rate bias variation over temperature, which, due to its non-linear nature, is difficult to compensate.
GB2322196A discloses a vibrating structure gyroscope with eight compliant legs supporting a resonator and carrying electromagnetic drive means and sensors. The actuator and sensor tracks are on a common leg and are isolated from each other by intermediate tracks. This feature tends to increase the width of the legs.
WO0120257A discloses a vibrating structure gyroscope with eight compliant legs supporting a resonator and piezoelectric actuators and sensors. In one embodiment however, magnetic transducers are employed with pairs of legs carrying the transducer conductors and straddling nodes of the resonator. However the legs are not symmetric.
GB 2276976A discloses A vibrating structure gyroscope, including:                a substantially planar annular resonator;        a substrate;        at least one drive transducer arranged to cause the annular resonator to oscillate in a primary in-plane mode at the resonant frequency of the primary mode and having an associated flexible support including first and second leg members which have a radially-extending line of symmetry (LS) between them;        a support arrangement including a plurality of flexible supports arranged to support the annular resonator from the substrate and to allow the annular resonator to oscillate in the primary in-plane mode and to oscillate in a secondary in-plane mode in response to an angular velocity applied around an axis substantially perpendicular to the plane of the annular resonator, and        at least one pick-off transducer arranged to detect oscillation of the annular resonator in the secondary in-plane mode and having an associated flexible support including first and second leg members which have a radially-extending line of symmetry between them;        wherein a magnetic field is arranged perpendicular to the plane of the annular resonator such that the drive transducers are located within the magnetic field,        wherein said at least one drive transducer includes a metal track on the annular resonator and on the flexible supports, and has a continuous metal track comprising a first track section running from the substrate along the first leg member, along a section of the annular resonator and along the second leg member back to the substrate, said first and second leg members only carrying the metal track associated with a single drive transducer,        